1. Field of the Invention
The present invention relates to an apparatus for converting digital image data into a visible image, and more specifically, to an apparatus for rasterizing image data comprising outlines of image portions.
2. Discussion of the Related Art
An apparatus for the conversion of digital image data into a visible image is used in reproduction means, such as printers, In such cases the digital image data is often available in the form of outlines which form the edges of image portions which are to be reproduced black. Usually, however, a reproducing means form its visible image by placing an imaginary raster of image elements, i.e., pixels, over the image and coloring some of the pixels black and the remainder white, Converting outline data into a pattern of black and white pixels is termed rasterizing. The pixel raster is usually orthogonal and thus has two major directions perpendicular to one another.
On rasterizing, pixels which are situated entirely within an image portion are assigned a binary value, e.g. 1, while the pixels which are situated entirely outside an image portion are given the other binary value, in this case 0. Pixels intersected by the outline of an image portion are given a value which, when the rasterized image data is printed in a printer, is as close as possible to the configuration of that outline. An apparatus of this kind is known from European patent application EP-A 0 406 956. In the known apparatus, the area fraction situated within an image portion is determined for pixels which are situated on an outline, and the area fraction is converted, by rounding off, to a binary value (0 or 1) for each following pixel extending along the outline. The rounding-off error in this case, or at least a number based on the rounding-off error, is passed on to the next pixel on the outline, where it is counted in the area fraction of that pixel before a binary value is calculated for that pixel by rounding off. As a result, a tooth structure forms at places where the outline extends approximately parallel to one of the two major directions of the pixel raster and intersects a number of contiguous pixels. The pixels intersected by the outline alternately have the values 0 and 1, while they are flanked on one side by pixels all having the value 0 and on the other side by pixels all having the value 1. The flanking pixels, of course, are situated entirely on the inside and outside respectively of the image portion.
When a pixel pattern calculated in this way is reproduced by a printer or a display, the referenced tooth structure is not visible in a number of cases. In many laser printers, the light spot by means of which the image is formed extends over a number of pixels and thus flattens out details of the order of magnitude of one pixel. This flattening effect results in the boundary line shifting somewhat on the print and ending up, for example, about midway between the pixels situated on the outline. In other printers or in displays which accurately reproduce the pixels, the visibility of the separate pixels is still always dependent upon the resolution of the pixel raster. If this is greater than approximately 20 pixels per mm, separate pixels can no longer be perceived as such with the naked eye and the tooth structure referred to also simply results in a shift of the perceived image boundary. By using this effect, it is possible, with the aid of different tooth structures, to position a reproduced boundary line more accurately than in steps of the size of one pixel. This known technique is termed "half bitting".
A disadvantage of the known apparatus is that the rasterization of an image follows the outlines and thus moves in arbitrary directions over the image. As a result, the rasterized image data becomes available in accordance with the pattern of the outlines, while when the data is used it is preferable to offer the data in the form of consecutive parallel image lines in one of the two major directions of the pixel raster. This is associated with the further use of the rasterized image data, such as by direct conversion thereof to control signals for a reproducing system, which usually operates image line-wise, for storage thereof in a memory. In the latter case, the binary pixel values are preferably stored in consecutive memory words of 32 bits, for example, for each image line, each bit containing one pixel value. The storage of the pixel values is, of course, most rapid if the pixel values are also supplied in the form of image lines. It is therefore desirable for the rasterization apparatus to deliver the binary pixel values image line-by-image line.